As intermediate paint, top paint and the like for outer panels of automobiles, heretofore water-based paint comprising a base resin such as acrylic resin, polyester resin and the like which contain functional groups such as hydroxyl or carboxyl groups; and melamine resin as a crosslinking agent has been frequently used.
As the melamine resin, water-soluble melamine resins of relatively low molecular weight which excel in dispersibility in water have been predominantly used, but recently investigations are made for use of hydrophobic melamine resins of relatively high molecular weight. While water-based paints containing such hydrophobic melamine resins exhibit higher coating film performance such as water resistance, compared to water-based paints containing water-soluble melamine resins, they have a problem that the films they form occasionally are inferior in smoothness or distinctness of image, due to low compatibility between the hydrophobic melamine resins with water which serves as the medium.
JP 2002-308993A discloses that aqueous resin dispersions containing the reaction product obtained by heat-treating specific acrylic resin, hydrophobic melamine resin and polyester resin under specific conditions show good dispersibility in water and can form coating film of excellent appearance. However, such aqueous resin dispersions are disadvantageous in respect of energy saving because the heating step is required at their production time, and furthermore coating films they form are in cases inferior in smoothness.
JP Sho 63 (1988)-193968A discloses that water-based coating compositions, which contain a crosslinking agent formed by dispersing hydrophobic melamine resin in water in the presence of a water-soluble resin, can be advantageously utilized as water-based base coat paint compositions which show excellent storage stability, can form coating films free of such defects as sagging and unevenness, and have good coating workability. However, coating films formed of the water-based coating compositions occasionally are inferior in smoothness and distinctness of image.
JP Hei 7 (1995)-41729A discloses aqueous resin dispersions formed by dispersing a graft resin having a weight-average molecular weight of 5,000-50,000, an acid value greater than 20 and hydrophilic side chains, and a hydrophobic melamine resin in an aqueous medium, excel in storage stability, heat stability, mechanical stability and so on. However, coating films formed with use of the water-based paint compositions containing the aqueous resin dispersions are inferior in smoothness and distinctness of image in certain occasions.